Simple Schematic Documentation That Aids Collaboration

August 12, 2024

Cadence PCB Solutions

Schematics for an electronic device are used to show circuit drawings as the basis for a PCB layout. While schematics do need to be used to define electrical connections between components and circuits, schematics have a deeper purpose: to show the documentation required to understand a design. This is always the type of front-end documentation needed to fully understand a design’s core functions, past revisions, and high level connections between components.

If you want to accurately communicate this in a single document, rather than spreading requirements across multiple documents, there are some simple pieces of documentation you can include in your schematics. There is a lot of benefit to this when working with collaborators on a multidisciplinary engineering team, and this is an approach larger companies take when working on complex products. Here is some of the documentation beyond circuit diagrams you can use in your schematics.

Core Documentation Beyond Circuit Diagrams

Normally, documentation that would be needed in product development is compiled into an engineering requirements document. Including some of this data in your schematic does not preclude an engineering requirements document, but it does make it much easier for collaborators to understand your design’s functions. With that in mind, there is some simple documentation you can hand draw or import into your schematics to create a much more detailed view of your design, such as:

Table of contents
Block diagrams
Power and timing sequencing
Revision history

This might sound like a simple piece of documentation, but a table of contents becomes very useful once a design starts to grow in size. A table of contents is not needed in a design with just a few schematic sheets, but complex designs with many components and a large number of nets will require a large number of schematic sheets. It is not uncommon to see designs with dozens of schematic sheets, and keeping track of each sheet in the schematic can be difficult.

If you decide to create a table of contents for a design, use it as the first page of the schematic. Inside your schematic capture tool, number each sheet sequentially in the order you want them to appear in a schematic print. Once you export the schematics to a PDF file, the table of contents will be automatically included and the numbering will match correctly.

pcb schematics documentation

Block Diagrams

A block diagram is a great way to see connections between each of the subsystems in an electronic device. Very large systems, rigid-flex PCBs, or systems with multiple circuit boards might need multiple block diagrams to adequately communicate connectivity and functionality. Make sure to include these as they help guide a reviewer, developer, or another engineer in understanding your system.

There are no set rules for drawing block diagrams, but typically the diagram will feature some of the following elements:

Main components that drive functionality
Groups of connections or specific interfaces, such as SPI or I2C
Sheet names or page numbers that contain circuitry in important blocks
Connectors that function as inputs and outputs
Color coding to distinguish signals, power, and certain groups of functional blocks

Remember, you have the freedom to draw your block diagram however you see fit, as long as it communicates the information required to understand a design’s architecture.

pcb schematics documentation

Example power supply section block diagram. (Source: Ultra Librarian)

Power and Timing Sequencing

Some systems will have timing or sequencing requirements related to power, peripheral bring up, or both. This is certainly the case in a device that has multiple regulators that spread power throughout the system, such as an embedded system or a complex power system. Some RF devices also require power sequencing on portions of the system, such as power amplifiers or oscillators.

Including a power sequencing and timing diagram and helps a systems architect know what to expect when a system is powered on. This can also help a test engineer understand correct functionality when designing manual or automated testing. A power sequencing diagram should clearly show the order in which regulators power on. A more detailed option will show time offsets between different regulators and the final voltage levels of the regulators involved in power up.

pcb schematics documentation

Revision History

Engineers who create fabrication and assembly drawings should be very familiar with revision blocks as they are commonly included in these drawings. Revision history can also be included in electrical schematics and they are very useful for understanding the change history for a project. Revision histories should be high level; they do not need to list every single change to each component, but rather the high level changes implemented in the device architecture.

Change logs should include the:

The date the change was applied
The design revision in which the change was implemented
The engineer responsible for implementing the change
The person who reviewed and/or approved the change

The revision number should match the build revision which implemented the change. In addition, it can be a good idea to include a build date in the schematics change list, which will essentially mark the end of tracking changes in a revision. The changes being tracked in the schematic change log should only include changes to circuitry that were later updated in the PCB. This change log documentation could be different from note entries included in the company’s VCS.

Jumper Settings

Jumpers are very useful for making a PCB reconfigurable, both during test and on-the production line. They are also useful for debugging a design before finalizing for mass manufacturing. These are normally placed as pin headers with an input into a center pin, such as shown below.

Jumper settings should be included in schematics so that it is clear how a circuit is affected by the jumper configuration. At minimum, a default setting that would be applied during normal system conditions and placed in assembly should be included in the schematic. This can be drawn directly in the symbol (see below) or placed as a note next to the symbol. Make sure the jumper is also included in the design’s assembly BOM and that any jumper placement instructions are included in documentation for the assembler.

pcb schematics documentation

Peripheral Addressing Notes

In embedded systems, some peripherals will require addressing that can be applied with configuration resistors, or sometimes with jumpers. These will normally pull a device to high or low voltage to set a specific address for an interface, such as I2C. This can also be applied to buses with specialty logic interfaces, such as parallel buses configured with GPIOs from an MCU/FPGA.

The default jumper or resistor settings should be included in the note alongside the address that these settings will impose. If resistors are used, the configuration is normally applied by setting some of the resistors to DNP. Make sure to include this note next to those resistors as part of the default settings, and include the alternative settings that will set a new desired address.

pcb schematics documentation

A variant of this is the approach to making a PCB configurable is the application of a solder bridge or a cut trace. The same documentation for jumpers should also be included. In the case of solderable bridges that would be used for configuration, it’s a good idea to state what is the resistor value that would be soldered onto a solder bridge pad to set a configuration state.

Another option is to include an address table that lists the addresses for the configurable components. An example I2C address table is shown below, where the included addresses are assumed to be static.

pcb schematics documentation

PCB Layout Notes

Finally, we have PCB layout notes that can be applied in a schematic sheet. Layout notes provide two functions: they guide the layout engineer on how/where to place specific parts, and they provide guidance for the design engineer who will be performing a design review.

Layout guidance should always be specific, listing which designator needs to be placed in a specific location in the PCB layout. This is most often applied to connectors, mounting holes, or large elements like heat sinks. These notes could also be applied to fans, fasteners, LEDs, or sensors.

pcb schematics documentation